It is often desirable to measure or compare two fluid pressure signals to ascertain disparity of a predetermined amount therebetween. If such disparity does occur, it is desirable to isolate the failed system from the remainder of the normally functioning system and to provide a signal which indicates such occurrence. The signal thus developed may be utilized for warning purposes or to operate additional equipment for a desired end result. It is further desired that once such disparity occurs the comparing apparatus is not permitted to return to a position such that the disparity indicating signal may be eliminated, that is the comparing apparatus should be latched once the disparity has been detected.
Prior art apparatus for accomplishing such comparison and latching has, for the most part, included relatively complex electronic or electrical equipment and in those cases where fluid pressure systems have been involved, relatively complex mechanism has been required. In some instances, even though the complex equipment is not required, it has been detected that the equipment does not properly operate in all cases. Such improper operaton may result from differences in fluid leakage within the system as well as the possibility that system pressure may be applied to operational parts of the system resulting in damage thereto.
The best prior art known to applicant at the present time is U.S. Pat. Nos. 3,391,611, 3,406,702, 3,570,516, 1,986,084, 2,983,278.